It is desired that a time standard assembly for a global positioning system (GPS), such as for satellites, space vehicles, and the like, be able to withstand a high level electromagnetic disturbance, such as from a nuclear event, to operate for an extended period of time without ground control assistance, to operate within specified error limits over given periods of time, and to have a long mission life expectancy and high probability of precision performance.
An example of a time standard assembly that is conventionally used for a global positioning system (GPS) is illustrated in FIGS. 1-5. Referring to FIG. 1, an atomic frequency standard (AFS), or so-called "atomic clock", employs a physics package 10 which outputs a frequency signal based upon atomic resonance that is amplified by servo amplifier 11 and input to a voltage controlled oscillator (VCXO) 12. The VCXO provides an oscillator signal to the synthesizer section "A"which synthesizes a standard 10.23 MHz frequency signal F.sub.o. A digital control section "B" responsive to ground control inputs is used to generate C-field control inputs which are applied to the physics package 10. The C-field inputs induce frequency changes in the physics package 10 which, for example, correct for relativistic effects and clock offset and drift. A control signal is also fed back to the physics package 10 in a primary loop from the VCXO through an RF multiplier 13.
In FIG. 2, the 10.23 MHz frequency signal F.sub.o output from the AFS unit is input to a frequency synthesizer unit FSU containing dividers, mixers, filters, and a numerically controlled oscillator NCO which is used to "dither" the phase of the 10.23 MHz signal within a range of +/- e. The NCO also sends measurements of the fine phase differences between the dithered and undithered 10.23 MHz signals to a processor (described below) which calculates any required phase correction values and feeds them back to the NCO of the FSU.
As shown in FIG. 3, the dithered 10.23 MHz output (F.sub.o +/- e) from the FSU is input to a microprocessor data unit MDU, which encodes the dithered frequency signals into clock code signals XP.sub.i. Two sets of paired encoder registers X1A, X1B and X2A, X2B generate respective sets of clock codes X1 and X2 which are combined to generate the output encoded clock codes XP.sub.i (the P codes). The MDU generates a unit (X1) epoch signal based upon the dithered 10.23 MHz frequency signal to mark successive epochs of time tracked by the MDU. The MDU encoder registers are reset to predetermined states with each successive epoch counted. A unit clock count (Z-count) is maintained for each successive epoch. The P code generator can be made partially upset-proof by implementing the components of the MDU using CMOS SOS technology. Incorrect P codes can be generated however if spurious dithered 10.23 MHz signals are input due to an FSU upset. Improper signal edges entering the MDU registers will produce incorrect counts in these registers, thus causing improper P codes to be generated.
FIG. 4 shows the further processing by the MDU of the encoded clock codes into output encoded navigational clock data. The X1 epoch signal is used to generate a signal for resetting a data encoder with each epoch. The epoch reset signal and dithered 10.23 MHz signal are used by a XG code generator to generate codes XG.sub.i (t). The data encoder receives the epoch reset signal and the formatted P-code data. The output of the data encoder is combined with the XG.sub.i (t) code signal to produce the output C/A codes which are used as encoded navigational data for the space vehicle using the GPS time standard assembly.
FIG. 5 illustrates the phase correction processing used by the MDU of the conventional GPS time standard assembly to measure the coarse phase differences between the dithered and undithered 10.23 MHz signals between successive epochs. The MDU uses the undithered 10.23 MHz frequency signal to generate a reference epoch signal, which is compared to the X1 epoch signal by a phase meter in order to measure the coarse phase differences between the undithered and dithered 10.23 MHz frequency signals. As noted above, fine (dither) phase differences are measured at the FSU. The measured values for coarse and fine phase differences are sent to a processor in the MDU where they are compared to the expected phase difference values calculated from a dither algorithm. Phase correction commands are then sent by the processor to the NCO in the FSU if the measured phase values are incorrect. There is a limit to the magnitude of the phase error that can be corrected. If this limit is exceeded, code generation is aborted and non-standard (NS) codes are generated until corrective action is taken from ground control.
The circuitry of both the "A" and "B" sections of the AFS of the described conventional GPS is susceptible to large scale upsets, for example, those induced by nuclear events. Shielding the circuits will reduce the likelihood of an upset, but not to a level that is satisfactory. Introducing high "Q"circuits at the output of the synthesizer of section "A" to bridge upsets might not prevent phase discontinuities. Upsets to the circuitry of section "B", which controls the strength of the C-field of the physics package, can cause large frequency changes. This arrangement also requires considerable circuitry which reduces reliability. Clock and timing measurements may be recovered after an upset (i.e., no latch-up) but may not be recovered at the correct phase or at the correct epoch or Z count.
The conventional GPS FSU is also susceptible to upsets because it uses frequency synthesizers and other non-linear devices. Such non-linear devices can recover from an upset, but the 10.23 MHz signal may experience a phase jump and, during the recovery process, noise edges may be generated that will result in incorrect code generation in the MDU. Upsets will also cause improper dither and erroneous phase measurements by the numerically controlled oscillator NCO.
The P and C/A code generators of the MDU can be made partially upset-proof through the use of CMOS SOS technology. However, they can generate incorrect codes due to an FSU upset. As can be seen from FIG. 3, spurious inputs from the dithered 10.23 MHz signal can cause incorrect generation of the P codes. Improper edges entering the MDU registers will produce incorrect counts in these registers, thus causing improper P codes to be generated. Provision must thus be made to adjust the state of these registers. The C/A code generator illustrated in FIG. 4 can be similarly affected.
As to the phase detection logic in FIG. 5, upsets to the FSU of the GPS equipment will cause large phase differences which may exceed the limit of that which can be corrected. Subsequent generation of NS codes will be instituted, thus making the space vehicle useless for navigation, and thereby requiring that the problem be fixed with the help of ground control. Even if the processor is upset proof itself, it can "crash" either as a result of an upset to other circuits or as a result of errors caused by other factors. When processor "crashes" occur, the processor enters into a reset routine which attempts to restore operation. Until operation is restored, NS codes are generated. Since the phase differences are measured against the undithered output frequency signal from the AFS, it is also crucial for phase measurement that the AFS be upset proof, otherwise an upset might not be detected.